I built this an experiment and discovered something I should have realised right from the start.

SimplePlanes does not use normal physics to make planes fly.

The fact that the game has a 'Wings' section should have been a dead giveaway.

The wings on this are made from structural panels on top, with internal ribs.

If the game used normal flight physics this should have been enough to make it fly.

Nope, not even a bit.

I added panels from the Wing section (no control surfaces) and it flies.

So here it is, a 'demonstration model' that proves what I am talking about.

Take the flat panels under the wings off and you have a brick.

Put them back and it flies.

Some XML used, engine tweaked for just a fraction more power to keep the plane going up, however slowly, and the fuel tank, because fuel.

"NO CONTROLS!" Only throttle and landing gear, that is it.
I spent a bit of time carefully tweaking it to fly nicely all by itself.